Petroleum
is a complex mixture of organic liquids called crude oil and natural gas,
which occurs naturally in the ground and was formed millions of years
ago. Crude oil varies from oilfield to oilfield in colour and composition,
from a pale yellow low viscosity liquid to heavy black 'treacle' consistencies.

Crude
oil and natural gas are extracted from the ground, on land or under the
oceans, by sinking an oil well and are then transported by pipeline and/or
ship to refineries where their components are processed into refined products.
Crude oil and natural gas are of little use in their raw state; their
value lies in what is created from them: fuels, lubricating oils, waxes,
asphalt, petrochemicals and pipeline quality natural gas.

An
oil refinery is an organised and coordinated arrangement of manufacturing
processes designed to produce physical and chemical changes in crude oil
to convert it into everyday products like petrol, diesel, lubricating
oil, fuel oil and bitumen.

As
crude oil comes from the well it contains a mixture of hydrocarbon compounds
and relatively small quantities of other materials such as oxygen, nitrogen,
sulphur, salt and water. In the refinery, most of these non - hydrocarbon
substances are removed and the oil is broken down into its various components,
and blended into useful products.

Natural
gas from the well, while principally methane, contains quantities of other
hydrocarbons - ethane, propane, butane, pentane and also carbon dioxide
and water. These components are separated from the methane at a gas fractionation
plant.

These
consist of straight or branched carbon rings saturated with hydrogen
atoms, the simplest of which is methane (CH4)
the main ingredient of natural gas. Others in this group include ethane
(C2H6), and propane (C3H8).

Hydrocarbons

With
very few carbon atoms (C1 to C4)
are light in density and are gases under normal atmospheric pressure.
Chemically paraffins are very stable compounds.

Naphthenes

Naphthenes
consist of carbon rings, sometimes with side chains, saturated with
hydrogen atoms. Naphthenes are chemically stable, they occur naturally
in crude oil and have properties similar to paraffins.

Aromatics

aromatic
hydrocarbons are compounds that contain a ring of six carbon atoms with
alternating double and single bonds and six attached hydrogen atoms.
This type of structure is known as a benzene ring. They occur naturally
in crude oil, and can also be created by the refining process.

The
more carbon atoms a hydrocarbon molecule has, the "heavier"
it is (the higher is its molecular weight) and the higher is its the boiling
point.

Small
quantities of a crude oil may be composed of compounds containing oxygen,
nitrogen, sulphur and metals. Sulphur content ranges from traces to more
than 5 per cent. If a crude oil contains appreciable quantities of sulphur
it is called a sour crude; if it contains little or no sulphur it is called
a sweet crude.

Every
refinery begins with the separation of crude oil into different fractions
by distillation.

The
fractions are further treated to convert them into mixtures of more useful
saleable products by various methods such as cracking, reforming, alkylation,
polymerisation and isomerisation. These mixtures of new compounds are
then separated using methods such as fractionation and solvent extraction.
Impurities are removed by various methods, e.g. dehydration, desalting,
sulphur removal and hydrotreating.

Refinery
processes have developed in response to changing market demands for certain
products. With the advent of the internal combustion engine the main task
of refineries became the production of petrol. The quantities of petrol
available from distillation alone was insufficient to satisfy consumer
demand. Refineries began to look for ways to produce more and better quality
petrol. Two types of processes have been developed:

Because
crude oil is a mixture of hydrocarbons with different boiling temperatures,
it can be separated by distillation into groups of hydrocarbons that boil
between two specified boiling points. Two types of distillation are performed:
atmospheric and vacuum.

Atmospheric
distillation takes place in a distilling column at or near atmospheric
pressure. The crude oil is heated to 350 - 400oC
and the vapour and liquid are piped into the distilling column. The liquid
falls to the bottom and the vapour rises, passing through a series of
perforated trays (sieve trays). Heavier hydrocarbons condense more quickly
and settle on lower trays and lighter hydrocarbons remain as a vapour
longer and condense on higher trays.

Liquid
fractions are drawn from the trays and removed. In this way the light
gases, methane, ethane, propane and butane pass out the top of the column,
petrol is formed in the top trays, kerosene and gas oils in the middle,
and fuel oils at the bottom. Residue drawn of the bottom may be burned
as fuel, processed into lubricating oils, waxes and bitumen or used as
feedstock for cracking units.

To
recover additional heavy distillates from this residue, it may be piped
to a second distillation column where the process is repeated under vacuum,
called vacuum distillation. This allows heavy hydrocarbons with
boiling points of 450oC and higher to be separated
without them partly cracking into unwanted products such as coke and gas.

The
heavy distillates recovered by vacuum distillation can be converted into
lubricating oils by a variety of processes. The most common of these is
called solvent extraction. In one version of this process the heavy
distillate is washed with a liquid which does not dissolve in it but which
dissolves (and so extracts) the non-lubricating oil components out of
it. Another version uses a liquid which does not dissolve in it but which
causes the non-lubricating oil components to precipitate (as an extract)
from it. Other processes exist which remove impurities by adsorption onto
a highly porous solid or which remove any waxes that may be present by
causing them to crystallise and precipitate out.

Reforming

Reforming
is a process which uses heat, pressure and a catalyst (usually containing
platinum) to bring about chemical reactions which upgrade naphthas into
high octane petrol and petrochemical feedstock. The naphthas are hydrocarbon
mixtures containing many paraffins and naphthenes. In Australia, this
naphtha feedstock comes from the crudes oil distillation or catalytic
cracking processes, but overseas it also comes from thermal cracking and
hydrocracking processes. Reforming converts a portion of these compounds
to isoparaffins and aromatics, which are used to blend higher octane petrol.

Cracking
processes break down heavier hydrocarbon molecules (high boiling point
oils) into lighter products such as petrol and diesel. These processes
include catalytic cracking, thermal cracking and hydrocracking.

e.g.

A
typical reaction:

catalyst

C16H34

->

C8H18

+

C8H16

Catalytic
cracking is used to convert heavy hydrocarbon fractions obtained by
vacuum distillation into a mixture of more useful products such as petrol
and light fuel oil. In this process, the feedstock undergoes a chemical
breakdown, under controlled heat (450 - 500oC)
and pressure, in the presence of a catalyst - a substance which promotes
the reaction without itself being chemically changed. Small pellets of
silica - alumina or silica - magnesia have proved to be the most effective
catalysts.

The
cracking reaction yields petrol, LPG, unsaturated olefin compounds, cracked
gas oils, a liquid residue called cycle oil, light gases and a solid coke
residue. Cycle oil is recycled to cause further breakdown and the coke,
which forms a layer on the catalyst, is removed by burning. The other
products are passed through a fractionator to be separated and separately
processed.

Fluid
catalytic cracking uses a catalyst in the form of a very fine powder
which flows like a liquid when agitated by steam, air or vapour. Feedstock
entering the process immediately meets a stream of very hot catalyst and
vaporises. The resulting vapours keep the catalyst fluidised as it passes
into the reactor, where the cracking takes place and where it is fluidised
by the hydrocarbon vapour. The catalyst next passes to a steam stripping
section where most of the volatile hydrocarbons are removed. It then passes
to a regenerator vessel where it is fluidised by a mixture of air and
the products of combustion which are produced as the coke on the catalyst
is burnt off. The catalyst then flows back to the reactor. The catalyst
thus undergoes a continuous circulation between the reactor, stripper
and regenerator sections.

The
catalyst is usually a mixture of aluminium oxide and silica. Most recently,
the introduction of synthetic zeolite catalysts has allowed much shorter
reaction times and improved yields and octane numbers of the cracked gasolines.

Thermal
cracking uses heat to break down the residue from vacuum distillation.
The lighter elements produced from this process can be made into distillate
fuels and petrol. Cracked gases are converted to petrol blending components
by alkylation or polymerisation. Naphtha is upgraded to high quality petrol
by reforming. Gas oil can be used as diesel fuel or can be converted to
petrol by hydrocracking. The heavy residue is converted into residual
oil or coke which is used in the manufacture of electrodes, graphite and
carbides.

This
process is the oldest technology and is not used in Australia.

Hydrocracking
can increase the yield of petrol components, as well as being used
to produce light distillates. It produces no residues, only light oils.
Hydrocracking is catalytic cracking in the presence of hydrogen. The extra
hydrogen saturates, or hydrogenates, the chemical bonds of the cracked
hydrocarbons and creates isomers with the desired characteristics. Hydrocracking
is also a treating process, because the hydrogen combines with contaminants
such as sulphur and nitrogen, allowing them to be removed.

Gas
oil feed is mixed with hydrogen, heated, and sent to a reactor vessel
with a fixed bed catalyst, where cracking and hydrogenation take place.
Products are sent to a fractionator to be separated. The hydrogen is recycled.
Residue from this reaction is mixed again with hydrogen, reheated, and
sent to a second reactor for further cracking under higher temperatures
and pressures.

In
addition to cracked naphtha for making petrol, hydrocracking yields light
gases useful for refinery fuel, or alkylation as well as components for
high quality fuel oils, lube oils and petrochemical feedstocks.

Following
the cracking processes it is necessary to build or rearrange some of the
lighter hydrocarbon molecules into high quality petrol or jet fuel blending
components or into petrochemicals. The former can be achieved by several
chemical process such as alkylation and isomerisation.

Olefins
such as propylene and butylene are produced by catalytic and thermal cracking.
Alkylation refers to the chemical bonding of these light molecules with
isobutane to form larger branched-chain molecules (isoparaffins) that
make high octane petrol.

Olefins
and isobutane are mixed with an acid catalyst and cooled. They react to
form alkylate, plus some normal butane, isobutane and propane. The resulting
liquid is neutralised and separated in a series of distillation columns.
Isobutane is recycled as feed and butane and propane sold as liquid petroleum
gas (LPG).

Isomerisation
refers to chemical rearrangement of straight-chain hydrocarbons (paraffins),
so that they contain branches attached to the main chain (isoparaffins).
This is done for two reasons:

they
create extra isobutane feed for alkylation

they
improve the octane of straight run pentanes and hexanes and hence make
them into better petrol blending components.

Isomerisation
is achieved by mixing normal butane with a little hydrogen and chloride
and allowed to react in the presence of a catalyst to form isobutane,
plus a small amount of normal butane and some lighter gases. Products
are separated in a fractionator. The lighter gases are used as refinery
fuel and the butane recycled as feed.

Pentanes
and hexanes are the lighter components of petrol. Isomerisation can be
used to improve petrol quality by converting these hydrocarbons to higher
octane isomers. The process is the same as for butane isomerisation.

Under
pressure and temperature, over an acidic catalyst, light unsaturated hydrocarbon
molecules react and combine with each other to form larger hydrocarbon
molecules. Such process can be used to react butenes (olefin molecules
with four carbon atoms) with iso-butane (branched paraffin molecules,
or isoparaffins, with four carbon atoms) to obtain a high octane olefinic
petrol blending component called polymer gasoline.

A
number of contaminants are found in crude oil. As the fractions travel
through the refinery processing units, these impurities can damage the
equipment, the catalysts and the quality of the products. There are also
legal limits on the contents of some impurities, like sulphur, in products.

Hydrotreating
is one way of removing many of the contaminants from many of the intermediate
or final products. In the hydrotreating process, the entering feedstock
is mixed with hydrogen and heated to 300 - 380oC.
The oil combined with the hydrogen then enters a reactor loaded with a
catalyst which promotes several reactions:

hydrogen
combines with sulphur to form hydrogen sulphide (H2S)

nitrogen
compounds are converted to ammonia

any
metals contained in the oil are deposited on the catalyst

some
of the olefins, aromatics or naphthenes become saturated with hydrogen
to become paraffins and some cracking takes place, causing the creation
of some methane, ethane, propane and butanes.

The
hydrogen sulphide created from hydrotreating is a toxic gas that needs
further treatment. The usual process involves two steps:

the
removal of the hydrogen sulphide gas from the hydrocarbon stream

the
conversion of hydrogen sulphide to elemental sulphur, a non-toxic and
useful chemical.

Solvent
extraction, using a solution of diethanolamine (DEA) dissolved in water,
is applied to separate the hydrogen sulphide gas from the process stream.
The hydrocarbon gas stream containing the hydrogen sulphide is bubbled
through a solution of diethanolamine solution (DEA) under high pressure,
such that the hydrogen sulphide gas dissolves in the DEA. The DEA and
hydrogen mixture is the heated at a low pressure and the dissolved hydrogen
sulphide is released as a concentrated gas stream which is sent to another
plant for conversion into sulphur.

Conversion
of the concentrated hydrogen sulphide gas into sulphur occurs in two stages.

Combustion
of part of the H2S stream in a furnace, producing sulphur
dioxide (SO2) water (H2O)
and sulphur (S).

2H2S

+

2O2

->

SO2

+

S

+

2H2O

Reaction
of the remainder of the H2S with the combustion
products in the presence of a catalyst. The H2S
reacts with the SO2 to form sulphur.

2H2S

+

2O2

->

3S

+

2H2O

As
the reaction products are cooled the sulphur drops out of the reaction
vessel in a molten state. Sulphur can be stored and shipped in either
a molten or solid state.

Preserving
air quality around a refinery involves controlling the following emissions:

sulphur
oxides

hydrocarbon
vapours

smoke

smells

Sulphur
enters the refinery in crude oil feed. Gippsland and most other Australian
crude oils have a low sulphur content but other crude's may contain up
to 5 per cent sulphur. To deal with this refineries incorporate a sulphur
recovery unit which operates on the principles described above.

Many
of the products used in a refinery produce hydrocarbon vapours. The escape
of vapours to atmosphere are prevented by various means. Floating roofs
are installed in tanks to prevent evaporation and so that there is no
space for vapour to gather in the tanks. Where floating roofs cannot be
used, the vapours from the tanks are collected in a vapour recovery system
and absorbed back into the product stream. In addition, pumps and valves
are routinely checked for vapour emissions and repaired if a leakage is
found.

Smoke
is formed when the burning mixture contains insufficient oxygen or is
not sufficiently mixed. Modern furnace control systems prevent this from
happening during normal operation.

Smells
are the most difficult emission to control and the easiest to detect.
Refinery smells are generally associated with compounds containing sulphur,
where even tiny losses are sufficient to cause a noticeable odour.

The
majority of the water discharged from the refinery has been used for cooling
the various process streams. The cooling water does not actually come
into contact with the process material and so has very little contamination.
The cooling water passes through large "interceptors" which
separate any oil from minute leaks etc., prior to discharge. The cooling
water system at Geelong Refinery is a once-through system with no recirculation.

Rainwater
falling on the refinery site must be treated before discharge to ensure
no oily material washed off process equipment leaves the refinery. This
is done first by passing the water through smaller "plant oil catchers",
which each treat rainwater from separate areas on the site, and then all
the streams pass to large "interceptors" similar to those used
for cooling water. The rainwater from the production areas is further
treated in a Dissolved Air Flotation (DAF) unit. This unit cleans the
water by using a flocculation agent to collect any remaining particles
or oil droplets and floating the resulting flock to the surface with millions
of tiny air bubbles. At the surface the flock is skimmed off and the clean
water discharged.

Process
water has actually come into contact with the process streams and so can
contain significant contamination. This water is treated in the "sour
water treater" where the contaminants (mostly ammonia and hydrogen
sulphide) are removed and then recovered or destroyed in a downstream
plant. The process water, when treated in this way, can be reused in parts
of the refinery and discharged through the process area rainwater treatment
system and the DAF unit.

Any
treated process water that is not reused is discharged as Trade Waste
to the sewerage system. This trade waste also includes the effluent from
the refinery sewage treatment plant and a portion of treated water from
the DAF unit.

As
most refineries import and export many feed materials and products by
ship, the refinery and harbour authorities are prepared for spillage from
the ship or pier. In the event of such a spill, equipment is always on
standby at the refinery and it is supported by the facilities of the Australian
Marine Oil Spill Centre at Geelong, Victoria.

The
refinery safeguards the land environment by ensuring the appropriate disposal
of all wastes.

Within
the refinery, all hydrocarbon wastes are recycled through the refinery
slops system. This system consists of a network of collection pipes and
a series of dewatering tanks. The recovered hydrocarbon is reprocessed
through the distillation units.

Wastes
that cannot be reprocessed are either recycled to manufacturers (e.g.
some spent catalysts can be reprocessed), disposed of in EPA-approved
facilities off-site, or chemically treated on-site to form inert materials
which can be disposed to land-fill within the refinery.

Waste
movements within the refinery require a "Process liquid, Sludge and
Solid waste disposal permit". Wastes that go off-site must have an
EPA "Waste Transport Permit".